This invention is concerned with a solid state device that senses gases at room temperature.
Traditional gas sensors based on semiconducting oxides must be operated at high temperature to cause thermal desorption of the chemisorbed gas on the surface of the oxide. The thermal desorption allows an equilibrium to be established between the gaseous species in the ambient environment and on the surface of the semiconducting oxide. The amount of adsorbed gas affects the resistance of the oxide; therefore, by monitoring the resistance, the ambient gas concentration can be determined.
The shortcomings of this approach arise out of the requirement for heating the sensor material. The temperature must be high enough to cause a reasonable response rate, i.e., if the temperature is too low, then the thermal desorption will be too slow. Prolonged operation at high temperature, however, can cause irreversible changes in the electrical properties of the oxide, which in turn can cause drift in the measurements.
The heating requirement also leads to problems with size, power and control. The heater controller can require a considerable amount of power and introduces additional expense because of the need to provide a means for maintaining temperature stability.
Electrochemical cells which operate at room temperature are available for measuring gas concentration. These cells, however, tend to exhibit continuous drift of their baseline and need frequent recalibration for this reason. This drift occurs due to a chemical buildup on the reference electrode.